Not having a problem with the guppies really, but my pH is sitting 6.8 and i've heard they like it closer to 7.4. I could be misinformed.
6.8 is RO water.
Are you testing the water after it sits overnight and the gas exhchanges?
7.4 would be the low end for me on any tank. The KH in my opinion should be at least 3 dKH, This puts the pH somewhere in the 7.4-7.8 range depending on a couple of other factors. If your tank tests at 6.8 after it has circulated and aireated overnight, I would worry about the KH level of your source water. KH is needed to counteract biological processes and keep the tank stable. If your tap is low Kh then you should supplement somehow, If your tap is not low Kh but your tank is then you need to increase partials to a level that maintains tank and tap at the same levels.
That still has little to do with the GH or TDs levels which are in fact what you are after.
Crushed coral will still give you both KH and GH, and will buffer the water nicely for your guppies.
With Guppies water parrameters (Those discussed here) don't matter much.
With any fish pH really makes no difference. This is a myth based on natural water supplies that does not stand up to testing despite its popularity in the hobby.
Marine salt would make a difference, and contains many of the ions tht your guppies would like, but it also contains some signifacant quantity of Sodium Chloride which would make no sense at all in a freshwater tank.
Dave